fix: preserve percent-encoding in external markdown links

[maruthang]

Fixes #245128

Summary

Bug: Ctrl+clicking a markdown link with a text fragment (e.g. https://ffmpeg.org/ffmpeg-all.html#:~:text=%2Dversion) incorrectly decodes percent-encoded characters, breaking the URL.

Root Cause: The MdLinkOpener constructs a vscode.Uri via vscode.Uri.from() for all link types, including external URLs. The Uri constructor decodes percent-encoded characters (e.g. %2D to -, %2F to /) and re-encodes them differently, which breaks text fragment directives and other encoding-sensitive URL components.

Fix: For external links, pass the original linkText string directly to the vscode.open command instead of constructing a vscode.Uri. The vscode.open command already supports string arguments, so the URL is opened with its original encoding preserved.

Changes

• extensions/markdown-language-features/src/util/openDocumentLink.ts: Handle external links before URI construction by passing the raw linkText string to vscode.open, preserving percent-encoding
• extensions/markdown-language-features/src/test/openDocumentLink.test.ts (new): Regression tests verifying that percent-encoding is preserved in text fragments, query strings, and Firebase-style URLs

Test Plan

• Added regression test that verifies vscode.Uri decodes %2D in fragments (demonstrating the bug)
• Added test for percent-encoding preservation in query strings
• Added test for Firebase-style URLs with %2F in paths
• Existing tests should continue to pass as internal/file links are unaffected